Plants that have a majority of elemental mercury emissions in the discharge flue gas (or waste gas) may utilize halogen additives to control mercury emissions. One primary effect of a halogen additive is to promote oxidized mercury species (Hg++) in the flue gas. Mercury emissions are generally reduced by two mechanisms: (a) adsorption of mercury (Hg++ in particular) on particles or particulates in the flue gas and subsequent removal by the particulate control device; and/or (b) absorption of Hg++ in a flue-gas desulfurization (“FGD”) scrubber. Among halogens, bromine and bromide additives and brominated sorbents are widely employed for mercury control from coal-fired sources. However, there are serious emerging problems associated with the use of high concentration bromide additives and sorbents. Applied as coal additives, iodine and iodide salts are an alternative to bromine.
There is a need to optimize performance and reduce operating costs of iodine additives for mercury control for plants with wet FGD scrubbers. Iodine or iodide compounds are often added onto the coal feed at a rate of about 1 to 30 ppmw of coal feed. Iodine is a relatively expensive compared to bromine, but is about 10 times more efficient as a mercury oxidizer. Compared to bromine and bromide additives, iodine and iodide salts can have fewer detrimental side effects, such as metal corrosion, and has less potential to create emissions of stratospheric ozone-destroying precursors (due in part to its use at much lower concentrations).
There is also a need to control iodine buildup in flue gas treatment processes, even where iodine and/or iodide salts are not added. Many coals used in utility plants have higher native iodine concentrations (>3 ppmw). The majority of North American coals and lignites (for which there is measured iodine data) have low iodine concentration (<1 ppmw). However, iodine is not comprehensively monitored in North American coals. Iodine is known to accumulate in certain coal formations such as marine roof coals and in select coals associated with volcanic activity.
Native iodine in the coal and/or iodine added to the coal feed is believed to first form hydroiodic acid (HI) as it cools after combustion. Further reactions convert a portion of this to molecular I2 gas. Iodide in flue gas as hydroiodic acid is water soluble and will be retained in the scrubber slurry. Iodine is less soluble in solution, but solubility is increased in a mixed iodide/iodine solution. In solution, the molecular iodine reacts reversibly with the negative I− ion, generating the I3− anion, which is soluble in water.
An excess of iodine dissolved in scrubber solution can not only be volatilized during upset conditions, leading to a characteristic “purple plume” stack emission but also interfere with acid gas removal. Iodine is an oxidation inhibitor. High concentrations of iodide/iodine in the scrubbing solution moderate the sulfite oxidation rate and suppress the oxidation reduction potential (ORP). Iodide is able to reduce either the sulfite or the peroxomonosulfate radicals or the catalytically active transition-metal ions and is thereby oxidized to iodine. It can subsequently be reduced to iodide again by excess sulfite. Therefore, iodine is able to inhibit the overall SO2 to sulfating reaction(s) and is not consumed in the process.